The molecular composition and chemistry of high-molecular weight multicomponent mixtures is difficult to study. As used to herein, "high-molecular weight" refers to those molecules possessing a molecular weight over about 250 Dalton (atomic mass units). As molecular weight increases, the number of possible structures increases exponentially and, at some point, the ability to identify individual constituents breaks down. The problem is compounded by the fact that as molecular weight increases, volatility decreases, thereby limiting the application of techniques such as gas chromatography to separate components by their thermodynamic properties in the unassociated (ideal) state. The ability to extend such techniques to higher molecular weight is further limited by the thermal stability of the molecules which change structures at excessive temperatures. Thus, a limitation of the analysis of higher-molecular weight components is that such studies must be conducted on these systems in the condensed state.
In the condensed state, two or more molecules may associate by electronic attractions (polar, van der Waars, electron donor-acceptor, etc.) into clusters of molecules. The total number of monomers plus the total number of clusters is defined as the total number of particles. When association occurs, the number of particles is reduced and the average size of the particles is increased compared to the totally dissociated (monomeric) state.
The association of molecules in the condensed state adds a new complication to the study of composition and chemistry. Intermolecular associations interfere with separations and cause behavioral changes that can mask the true molecular composition.
That such behavior has served to confuse the scientific community about the composition and chemistry of high-molecular weight species, can be readily shown by examination of the published literature regarding asphaltenes. (See Bunget, J. W. and Li, N. C., Chemistry of Asphaltenes, Advances in Chemistry Series, ACS, 195 (1981); Winniford, R. S., "The Evidence for Association of Asphaltenes in Dilute Solutions," J. of Inst. of Pet., 49, pp. 215-221, July (1963); Speight, J. G., "Influence of Temperature and Solvent on the Precipitation of Asphaltenes," Fuel Science and Tech. Interntl., 8, 6 (1990); Yen, T. F., "The Colloidal Aspects of a Macrostructure of Petroleum Asphalt", Fuel Science Technology International, 10, 723-734 (1992); Cooper, A. R., Determination of Molecular Weight, Chemical Analysis Science, 103, John Wiley and Son, New York (1989).
Asphaltenes are defined as those species which precipitate from solution, and to date, there is no general agreement as to the structural features or even the size of the molecules which make up asphaltenes, even when the study is restricted to a common starting material.
The thermodynamics of association, whether self-association as in condensation or precipitation of like species, or intermolecular association as in condensation, precipitation or adsorption of unlike species strongly influence the liquid phase behavior. In spite of the general recognition of the importance of thermodynamics in these associations, there remains a need for a methodology to quantify these properties in a manner meaningful at the molecular level. The present invention describes a method that can be used to determine the condition of molecules in the associated state in thermodynamic terms generally recognized within the scientific community. The invention involves in part a mathematical method named the Bunget-Russell-Devineni Molecular Association Thermodynamic Method, or BRD method.